The invention relates to digital-to-analog converters, and more particularly to digital-to-analog converters that include very precise, externally accessible "output amplifier feedback resistors", and small sized, low cost bit current determining resistors, the sheet resistance of which can have manufacturing tolerances as high as .+-.30%.
Monolithic digital-to-analog converter (DAC) circuits are well known in the art, and typically include a plurality of bit switches each responsive to a particular bit within the digital input word for selectively steering an associated bit switch current to a current summing node to provide an analog output current. The current contributed to the analog output current by each of the bit switches is scaled in a binarily weighted fashion in accordance with the binary weighting of the particular bit to which that bit switch is responsive. Many such digital-to-analog converter circuits include an output amplifier into which the analog current flows via a current summing conductor. Such output amplifiers often have a feedback resistor. An analog output voltage that is produced by the output amplifier has a value proportional to the ratio of the resistance of a bit current determining resistor and the resistance of the feedback resistor. Consequently, if the feedback resistor has a very precise absolute value, then the current determining resistor ordinarily also must have a precise absolute value.
In certain applications of DACs, the terminals of the feedback resistors are externally accessible. Users often connect external resistances in parallel and/or in series with the feedback resistors to adjust the output voltage range of the DAC. In such instances, it is desirable for the feedback resistors to have an extremely precise value, so that the external range adjusting resistors have a predetermined value. However, if each of the feedback resistors has a precise absolute value, then each of the numerous bit current determining resistors ordinarily also must have a precise value. Precise resistors in an integrated circuit usually require a relatively large amount of chip area, which adds to the cost of the DACs.
Those skilled in the art know that relatively high accuracy, low cost monolithic digital-to-analog converters have been implemented wherein the absolute values of the resistances of feedback resistors and the bit current determining resistors are not precise, but the ratios between such feedback resistors and current determining resistors are precise. Customers relying upon precise absolute values of the feedback resistors have been unable to conveniently use such digital-to-analog converters.
Those skilled in the art will appreciate that any high accuracy digital-to-analog converter must have internal bit currents that are substantially independent of temperature over a specified temperature range for the digital-to-analog converter, and that a great deal of attention must be paid to this aspect of circuit performance as the DAC is designed. Typically, internal voltage reference circuits are provided that compensate for the thermal drift in the base-to-emitter voltages of the transistors that produce the base voltage applied to the current source transistors, which thereby produce the voltages across the bit current determining resistors, and thereby determine the bit currents.
Those skilled in the art know that nichrome resistors can be laser trimmed to produce very precise resistive voltage dividers. However, the use of laser trimmed voltage dividers in voltage reference circuits usually results in changes in current density in transistors that produce the reference voltage. Such changes in current density result in changes in the thermal drift of the emitter-to-base voltages of such transistors, and hence in the reference voltages. Consequently, use of laser trimmed resistive voltage dividers usually is avoided in circuitry in which voltage levels must be precisely compensated for thermal drift.
Those skilled in the art also know that it is highly desirable to avoid use of laser trimmed resistors in integrated circuits if very large percentage changes in resistance must be produced by laser trimming, because if a resistor is to be trimmable over a large range, it must be increased in chip area, obviously increasing the cost of the final product. Increase size of trimmable resistors also increases the amount of time required for laser trimming. Since laser trimming equipment is exceedingly expensive, laser trimming stations in an integrated circuit manufacturing facility tend to be "bottlenecks" in a production process. Obviously, designs that substantially increase the area of an integrated circuit chip and increase the amount of laser trimming are to be avoided if possible.
Previous DACs, mostly of hybrid construction, having precision, externally accessible feedback resistors have been provided, wherein the bit currents were trimmed or adjusted to precise values by means of an operational amplifier control circuit that produces the base reference voltage for the bit current source transistors. The operational amplifier in such circuits is adversely affected by parasitic signals produced by signal transitions in the bit switches because the DFM operational amplifier take a long time to settle from such parasitic signals.
Thus, there is an unmet need for a low cost, highly accurate, thermally stable digital-to-analog converter that provides externally accessible output amplifier feedback resistors having very precise absolute values of resistance, yet avoids the need for providing a large number of resistors that must be trimmed to produce a very large precentage change in resistance, and avoids complications caused by capacitive coupling of parasitic signals from the bit switches to reference voltage circuitry that biases the bit current determining resistors. There is also an unmet need for a practical integrated potentiometer. Some integrated potentiometers that are incrementally adjustable between several different resistances are known, such as the ones disclosed in U.S. Pat. Nos. 4,150,366 and 4,201,970. Such prior integrated potentiometers are not accurately enough trimmable to be very useful.